[Paper Review] Two Emission Mechanisms in the Fermi Bubbles: A Possible Signal of Annihilating Dark Matter

Dan Hooper, Tracy R. Slatyer|arXiv (Cornell University)|Feb 26, 2013

Dark Matter and Cosmic Phenomena18 citations

TL;DR

The paper proposes that the low-latitude Fermi Bubbles exhibit a distinct gamma-ray spectral feature peaking at 1–4 GeV, inconsistent with inverse Compton scattering from cosmic-ray electrons, and attributes this to a second emission mechanism. This component is spectrally and spatially consistent with a 10 GeV dark matter annihilation signal to leptons or 50 GeV to quarks, following a distribution steeper than NFW, offering a compelling dark matter interpretation of the extended Galactic Center excess.

ABSTRACT

We study the variation of the spectrum of the Fermi Bubbles with Galactic latitude. Far from the Galactic plane (|b| > 30 degrees), the observed gamma-ray emission is nearly invariant with latitude, and is consistent with arising from inverse Compton scattering of the interstellar radiation field by cosmic-ray electrons with an approximately power-law spectrum. The same electrons in the presence of microgauss-scale magnetic fields can also generate the the observed microwave "haze". At lower latitudes (b < 20 degrees), in contrast, the spectrum of the emission correlated with the Bubbles possesses a pronounced spectral feature peaking at 1-4 GeV (in E^2 dN/dE) which cannot be generated by any realistic spectrum of electrons. Instead, we conclude that a second (non-inverse-Compton) emission mechanism must be responsible for the bulk of the low-energy, low-latitude emission. This second component is spectrally similar to the excess GeV emission previously reported from the Galactic Center (GC), and also appears spatially consistent with a luminosity per volume falling approximately as r^-2.4, where r is the distance from the GC. We argue that the spectral feature visible in the low-latitude Bubbles is the extended counterpart of the GC excess, now detected out to at least 2-3 kpc from the GC. The spectrum and angular distribution of the signal is consistent with that predicted from ~10 GeV dark matter particles annihilating to leptons, or from ~50 GeV dark matter particles annihilating to quarks, following a distribution similar to the canonical Navarro-Frenk-White (NFW) profile. We also consider millisecond pulsars as a possible astrophysical explanation for the signal, as observed millisecond pulsars possess a spectral cutoff at approximately the required energy. Any such scenario would require a large population of unresolved millisecond pulsars extending at least 2-3 kpc from the GC.

Motivation & Objective

• To investigate the spectral variation of the Fermi Bubbles with Galactic latitude to identify deviations from standard inverse Compton emission.
• To determine whether the observed gamma-ray spectrum at low latitudes can be explained by a single electron population or requires a second emission mechanism.
• To test whether the low-energy spectral feature at 1–4 GeV is consistent with dark matter annihilation, particularly in comparison to the Galactic Center excess.
• To evaluate alternative astrophysical explanations, such as unresolved millisecond pulsars, and assess their viability.

Proposed method

• Anisotropic spectral analysis of Fermi-LAT data across different Galactic latitudes, focusing on |b| > 30° and |b| < 20°.
• Modeling of inverse Compton scattering from a power-law electron population (dNe/dE ∝ E−3) interacting with the interstellar radiation field.
• Estimation of synchrotron emission from the same electron population under microgauss-scale magnetic fields to match the microwave haze.
• Fitting the low-latitude gamma-ray spectrum with a component beyond inverse Compton, assuming a dark matter annihilation origin with specific final states and mass.
• Comparison of spatial morphology (luminosity per volume ∝ r−2.4) to NFW and steeper profiles to assess dark matter distribution compatibility.
• Evaluation of millisecond pulsar scenarios by requiring a dense, unresolved population extending 2–3 kpc from the Galactic Center.

Experimental results

Research questions

• RQ1 Can the nearly flat, hard-spectrum gamma-ray emission at high latitudes (|b| > 30°) be explained by inverse Compton scattering from a single electron population?
• RQ2 Why does the gamma-ray spectrum at low latitudes (|b| < 20°) exhibit a pronounced peak at 1–4 GeV not explainable by inverse Compton processes?
• RQ3 Is the spectral and spatial morphology of the low-latitude emission consistent with dark matter annihilation, particularly the extended counterpart of the Galactic Center excess?
• RQ4 Could millisecond pulsars produce the observed signal, and what population density would be required to explain the emission?
• RQ5 How does the observed luminosity profile (r−2.4) compare to theoretical dark matter profiles like NFW?

Key findings

• The high-latitude Fermi Bubbles (|b| > 30°) exhibit a nearly invariant, flat gamma-ray spectrum consistent with inverse Compton scattering from a power-law electron population (dNe/dE ∝ E−3).
• The same electron population, in the presence of microgauss-scale magnetic fields, naturally produces synchrotron emission matching the observed microwave haze, supporting a common origin.
• At low latitudes (|b| < 20°), a distinct spectral feature peaking at 1–4 GeV in E²dN/dE cannot be explained by inverse Compton scattering, indicating a second emission mechanism.
• The low-latitude signal is spatially consistent with a luminosity per volume declining as r−2.4, matching the extended Galactic Center excess and suggesting a common origin.
• The spectrum and morphology are broadly consistent with 10 GeV dark matter particles annihilating to leptons or 50 GeV particles annihilating to quarks, with a distribution steeper than the canonical NFW profile.
• A millisecond pulsar explanation would require an implausibly dense population of unresolved pulsars extending at least 2–3 kpc from the Galactic Center.